The invention relates to a photographic objective of the modified double gauss type comprising three lens elements (L1, L2, L3) and having the refractive power sequence positive, positive, negative, an aperture and a rear group including lens elements (L4, L5, L6) of refractive power sequence negative, positive, positive, wherein in the rear group the lens element of negative refractive power (L4) is designed as a meniscus with an aspheric surface concave relative to the aperture, and a lens element of negative refractive power (L7) with surfaces concave on both sides is arranged downstream of the last lens element of positive refractive power (L6).
Photographic objectives of the double gauss type are distinguished by an arrangement, symmetrical relative to the aperture, of lens elements with the refractive power sequence ++−/−++ or −++/++−. U.S. Pat. No. 6,519,097 B2 discloses such an objective with the refractive power sequence ++−/−++ in which all the lens elements are designed as individual lenses. The relative aperture of the objective is 1:2.9.
DE 22 22 892 C3 discloses an identical objective in which the two lens elements adjacent to the aperture are designed in each case as a cemented meniscus which is hollow toward the aperture. The objective has a relative aperture of 1:2 and exhibits a compact overall lens.
DE 40 05 300 C2 discloses a wide-angle objective of the modified double gauss type with the focal length f=35 mm, which exhibits a compact overall length, a good imaging quality and a high aperture of 1:1.4. The first lens is designed as a cemented component that is concave on the object side and has a positive refractive power overall. The two downstream lenses of positive and negative refractive power are likewise designed as cemented components, the surface that is convex on the object side being designed aspherically. A lens of negative and positive refractive power is embodied on the image side as a cemented component, a meniscus of positive refractive power that is concave on the object side being placed upstream of this cemented component. The further image-side lens of positive refractive power is cemented with a lens of negative refractive power, the object-side convex surface of this lens element being of aspheric design.
The concave front surface of the objective leads firstly to a widening of the incoming imaging beam which must be counteracted by appropriately strong refractive powers of the downstream lenses toward the aperture.
The two aspheric surfaces are relatively expensive to manufacture. Moreover, these surfaces are very susceptible to tilt errors and centering errors and therefore lead to an increased outlay on assembly during the manufacture of the cemented components and insertion into the objective system. The design of the first lens element as a cemented component raises the number of the lenses to be fabricated, and thereby likewise leads to a rise in cost.